Traffic flows near the interchanges that link Interstate 495 and Interstate 270 in Bethesda, Md. (Katherine Frey/The Washington Post)

Trump administration officials are preparing to issue a proposal within days to freeze fuel-efficiency standards for cars and light trucks for six years and challenge the right of California and other states to set their own tailpipe standards, according to four current and former federal officials. The move would amount to one of the biggest regulatory rollbacks of the Trump presidency.

The proposal, driven by a new analysis prepared by the Transportation Department’s National Highway Traffic Safety Administration (NHTSA), is likely to mark a sharp reversal from the findings NHTSA and the Environmental Protection Agency reached a year and a half ago in the waning days of the Obama administration.

In late 2016, the agencies concluded that stricter fuel-efficiency targets would save consumers money without compromising safety. Now, the current and former officials said, the government is poised to project that these goals would boost vehicle price tags and could endanger Americans by encouraging them to stick to driving older, less-safe cars and trucks.

Neither NHTSA nor the EPA responded to a request for comment Tuesday.

The joint proposal will lay out several options for setting miles-per-gallon standards for cars and light trucks over the next several years, according to four people who spoke on the condition of anonymity to discuss internal deliberations, but will recommend freezing them at Model Year 2020 levels through 2026. That translates into maintaining a fleetwide average of 35 mpg for six years, rather than raising it to about 50 mpg by 2025 under the rule.

In addition, the agencies will take comment on whether to block California’s right under the Clean Air Act to set its own emissions limit on cars and light trucks and require the sale of a certain number of electric vehicles in the state each year. Any effort to revoke California’s waiver to set its own standards would almost certainly spark a legal battle with the state and could have major implications for a dozen other states and the District, which have adopted California’s standards as their own.

Thomas J. Pyle, president of the conservative Institute for Energy Research, said in an email that Trump “is doing the right thing” in scaling back the emissions targets the agencies set in 2012.

“The Obama administration promised a review of their unrealistic mandate, but decided to lock it down after they didn’t like the outcome of the last election,” Pyle said. “While we would prefer a full repeal of the [current standards], the Trump proposal is a reasonable first step.”

Martha Roberts, a senior attorney at the Environmental Defense Fund, an advocacy group, said Tuesday that the car standards have been a “made-in-America success story” that helped reduce air pollution while saving people money at the gas pump.

If the administration goes ahead with its plan, Roberts said, it will mean many people will ultimately pay more because of higher fuel bills for less fuel-efficient vehicles. “It’s low-income, longer- commuting Americans that are particularly hit hard,” she said.

The latest proposal, which is similar to a version that The Washington Post and other outlets reported on in the spring, would undo one of President Barack Obama’s most significant climate policies and present a major challenge for automakers. Unless the administration successfully revokes the 2009 Clean Air Act waiver California received to set its own tailpipe standards, firms may face the prospect of producing vehicles that must meet different mileage targets depending on the state where they are sold.

California, 16 other states and the District are challenging the federal government’s push to revisit the existing emissions standards, which represent the first federal carbon limits on vehicles.

“The Trump administration’s assault on clean car standards risks our ability to protect our children’s health, tackle climate change, and save hard-working Americans money,” said California Attorney General Xavier Becerra when then-EPA Administrator Scott Pruitt said the agency would reconsider the rule.

This political and legal fight — one that has implications for how Americans drive, the viability of the nation’s auto industry and America’s carbon footprint — comes down to a numerical debate.

The name of the new proposal — the “Safer and Affordable Fuel Efficient (SAFE) Vehicles Rules” — reflects how administration officials have overhauled the standards and their emphasis. NHTSA estimates the technology required to meet existing efficiency targets would be at least twice as expensive as projected in 2016, and that Americans would drive their old cars so much longer as a result that it would imperil their safety.

In the 2016 evaluation, according to two technical experts, the EPA projected that hitting the model year 2025 targets compared with those in 2020 would add about $900 to the average cost of a vehicle. NHTSA found it would add a little over $1,200.

Under both scenarios, the agencies concluded, fuel savings would more than offset the higher purchase price.

But the new proposal pegs the additional cost at more than $2,000, according to two people briefed on the matter.

Affordability has been a central issue in the fight over vehicle-efficiency standards, with environmental advocates highlighting the fuel savings from more efficient cars and trucks, and some industry groups saying that higher purchase prices will sap buyers’ ability to purchase new vehicles.

“Barring a sudden and prolonged spike in fuel prices, new light-duty vehicle customers are, on average, unlikely to shift to more fuel-efficient vehicles and, in fact are likely to do the opposite,” the National Automobile Dealers Association wrote in a regulatory filing last year. Without higher gas prices, “the benefits associated with mandated higher fuel economy performance will not ‘pencil out’ for most customers.”

In its regulatory justification for the tighter tailpipe rules in 2012, federal officials pointed to overall savings on buying gas as a key economic benefit for owners of more efficient cars.

“While the initial purchase price of those vehicles will increase, the overall cost of owning them — including their operating costs — will decrease, because their fuel consumption will decline significantly,” they wrote.during the Obama administration

David Friedman, who directs cars and product policy and analysis at the Consumers Union and served as NHTSA’s acting administrator under the Obama administration, said that the agencies’ cost estimates are crucial when setting fuel-efficiency requirements because they determine what sort of future mileage goal is feasible.

“At the end of the day, Friedman said, “it’s the cost.”

brady.dennis@washpost.com

michael.laris@washpost.com